BTEB Refrigeration & AC Exam cheat sheet

Cooling capacity: Q = m × Δh, where m is refrigerant mass flow (kg/s) and Δh is enthalpy change across the evaporator (kJ/kg); 1 TR = 3.517 kW = 12,000 Btu/h = 3024 kcal/h.

Carnot COP for cooling = T_L / (T_H − T_L) with absolute temperatures in kelvin; actual COP is always lower, and COP falls as the condensing temperature rises or the evaporating temperature falls.

Sensible heat changes dry-bulb temperature only (Q = m × c_p × ΔT); latent heat changes moisture content without changing dry-bulb temperature; total heat = sensible + latent.

Pressure–enthalpy diagram reading: evaporation is a horizontal line at low pressure, condensation a horizontal line at high pressure, compression a curve rising to the right, expansion a vertical line at constant enthalpy.

Standard vapour-compression cycle order: compression, condensation, expansion, evaporation; superheat is added after full evaporation and subcooling after full condensation.

Subcooling increases refrigerating effect by lowering the enthalpy entering the evaporator; superheat protects the compressor but adds no useful refrigerating effect.

Refrigerant glide is the temperature difference between bubble and dew points of a zeotropic blend; for blends, evaporating and condensing pressures must be read at dew point for superheat and bubble point for subcooling.

Absolute pressure = gauge pressure + atmospheric pressure (about 101.3 kPa or 14.7 psi); all refrigerant tables and saturation data use absolute pressure.

Dalton's law: total pressure of a mixture equals the sum of partial pressures, which is why air in a system raises head pressure and discharge temperature.

The four essential components are compressor, condenser, expansion device and evaporator; a system missing any one cannot maintain a continuous refrigeration cycle.

Compressor types: reciprocating (positive displacement, high pressure ratio), scroll (fewer parts, quiet, common in small AC), screw and centrifugal (large capacities, centrifugal needs high evaporator pressure).

Thermostatic expansion valve controls superheat by balancing bulb pressure against evaporator pressure plus spring force; a starved evaporator gives high superheat, a flooded one gives low superheat.

Capillary tube systems are critically charged: charge quantity and tube length set the flow, so no adjustable superheat exists and any leak or overcharge seriously degrades performance.

Condenser types: air-cooled, water-cooled (shell-and-tube) and evaporative; heat rejected equals refrigerating effect plus compressor work, Q_cond = Q_evap + W_comp.

Evaporator types: flooded (filled with liquid, needs a surge drum or low-side float), direct-expansion (dry, fed by TXV) and forced-draft finned coils for air cooling.

Accessories: filter-drier removes moisture and particles from the liquid line, accumulator prevents liquid floodback on the suction line, oil separator returns lubricant to the compressor.

Liquid line sight glass shows bubbles when the charge is short or subcooling is inadequate; moisture-indicating crystals change colour when water is present.

Safety controls: high-pressure cut-out, low-pressure cut-out, oil-pressure switch with time delay, and motor overload/thermal protection.

Sensible heat ratio SHR = sensible load / total load; a low SHR means high latent load, requiring lower apparatus dew point or greater dehumidification capacity.

Cooling coil bypass factor BF = (leaving air DB − apparatus dew point) / (entering air DB − apparatus dew point); contact factor = 1 − BF.

Room sensible heat equation: Q_s = 1.21 × m³/s × ΔT (kW) or 1.08 × cfm × ΔT (Btu/h); latent heat: Q_L = 3010 × m³/s × Δw (kW) or 4840 × cfm × Δgrains.

Psychrometric properties: dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio, enthalpy and specific volume; wet-bulb equals dry-bulb only at saturation.

Outdoor design conditions are selected at a low exceedance percentile (commonly 0.4–1% for cooling), not at the absolute maximum recorded temperature.

Load components: conduction through walls, roof and glass; solar radiation through glazing; people (about 75 W sensible and 55 W latent per person at rest, more with activity); lighting; equipment; and outdoor air ventilation load.

Infiltration and ventilation: use air-change method or crack-length method for infiltration; ventilation air carries both sensible and latent load and is often the largest single component.

Diversity and safety factors: apply a modest margin (typically 5–10%) after summing loads; oversizing causes short cycling, poor dehumidification and higher energy use.

Latent load from people and infiltration must be met by coil dehumidification; if SHR is very high, sensible capacity dominates and humidity control may need reheat or dedicated outdoor air.

Window and split units are direct-expansion, air-cooled, typically rated in tons or Btu/h; EER = capacity (Btu/h) / power input (W), and SEER is the seasonal rating.

Chilled water systems use a chiller (air-cooled or water-cooled), pumps, AHUs or FCUs; a water-cooled chiller with cooling tower achieves lower condensing temperature and higher efficiency than an air-cooled chiller.

Air handling unit sections: filter, cooling coil, heating coil, humidifier, fan and mixing box; a draw-through arrangement places the fan after the coil, a blow-through arrangement places it before.

Fan laws: flow varies with speed, pressure with speed squared, power with speed cubed; halving speed cuts power to one-eighth.

Fan total pressure = static pressure + velocity pressure; velocity pressure = 0.5 × ρ × v², and fan power = airflow × total pressure / efficiency.

Duct systems: equal-friction and static-regain methods size ducts; velocity limits (about 4–6 m/s in main ducts) control noise and pressure drop.

Air distribution devices: diffusers, grilles and registers; throw, drop and terminal velocity determine comfort, and a ceiling diffuser should be selected for the required NC (noise criteria) level.

Cooling tower approach = leaving water temperature − entering wet-bulb temperature; range = entering water − leaving water; approach cannot be zero and typically runs 3–5 K.

Refrigerant piping practice: use dry nitrogen purge while brazing to prevent oxidation, install a filter-drier, and keep suction line insulated with vapour barrier.

Evacuation: pull a vacuum to at least 500 microns (0.67 mbar) and hold it for 10–30 minutes; a rising vacuum indicates moisture or a leak.

Triple evacuation: evacuate, break with dry nitrogen, repeat three times to remove moisture and non-condensables; never use refrigerant to blow out a system.

Leak testing: pressure test with dry nitrogen (typically 1.5 times working pressure) then soap solution or electronic leak detector; never use oxygen or compressed air.

Charging: liquid charging into the liquid line for blends; vapour charging into the suction line only for pure refrigerants; weigh-in method is most accurate.

Superheat setting for a TXV: measure suction pressure, convert to saturation temperature, subtract from suction line temperature; typical target 5–8 K at the evaporator outlet.

Subcooling check: liquid line temperature subtracted from condensing saturation temperature; target commonly 5–10 K, and low subcooling with bubbles suggests undercharge.

Commissioning records: airflow (balancing hood), static pressures, voltages and currents, superheat, subcooling, compressor amps versus nameplate, and filter pressure drop.

Electrical: verify supply voltage within ±10% of nameplate, correct phase rotation on three-phase units, tight terminals and correct earthing before energising.

Routine maintenance: clean condenser and evaporator coils, replace or clean filters, check refrigerant charge, tighten electrical connections, lubricate bearings, and inspect drain pans and insulation.

High discharge pressure causes: dirty condenser, non-condensables, overcharge, high ambient or cooling-water temperature, restricted condenser water flow, or fan failure.

Low suction pressure causes: undercharge, restricted liquid line or filter-drier, faulty TXV, frosted or blocked evaporator, or low load; high suction pressure points to overcharge, weak compressor valves, or excessive load.

Liquid floodback shows as a cold, sweating suction line and low superheat; liquid slugging causes compressor noise and damage - use an accumulator or correct the charge.

Compressor short cycling causes: low-pressure control set too close, thermostat differential too narrow, overload trip, or control wiring fault.

Safety: recover refrigerant, never vent to atmosphere; use PPE (goggles, gloves), ventilate the work area, and beware of asphyxiation and frostbite from liquid refrigerant.

Refrigerant cylinders must be stored upright, secured, below 50 °C, and never filled above 80% liquid by volume; use a scale for charging.

Electrical safety: lock out and tag out before servicing, discharge capacitors, verify zero voltage with a tester, and never bypass safety controls permanently.

Combustion and confined spaces: test for leaks, ensure ventilation, and use a calibrated gas detector before entering a plant room or pit.